Silicon carbide semiconductors exhibit higher dielectric breakdown voltages than those of silicon semiconductors as well as wide energy band gaps, and also have various excellent properties such as high thermal conductivity, and therefore their applications for light emitting devices, high power devices, high-temperature resistant devices, radiation resistant devices, high frequency devices, and the like have been expected.
In addition, silicon carbide semiconductors have been applied to Schottky barrier diodes. These silicon carbide (SiC) Schottky barrier diodes have conventionally been known to cause device breakdown even at a relatively low surge current when the surge current flows in the forward direction.
Thus, in order to solve this problem, a device structure in which n-type regions and p-type regions are arranged in parallel on one surface of a SiC semiconductor device so that the injection of holes that are minority carriers occurs from the p-type regions at the time of high current conduction has been proposed (for example, refer to Non-Patent Document 1). When such device structures are prepared, the surge resistance can be improved. Such device structures are known as a merged p-i-n Schottky (MPS) structure.
In the MPS structure, Schottky diodes and pn-type diodes are arranged alternately on one surface of a semiconductor device. Accordingly, on one surface of a semiconductor device, it is necessary to provide a junction layer constituted of a junction material that forms a favorable Schottky junction with an n-type semiconductor region and also forms a favorable ohmic junction with a p-type semiconductor region.
Incidentally, an aluminum-titanium (Al—Ti) alloy has been known as one of the metals to form an ohmic electrode with respect to p-type silicon carbide. In addition, it is accepted that an annealing process at a high temperature of 1,000° C. or higher is generally required in order to form an ohmic electrode having a low resistance. However, when an annealing process is carried out at a high temperature using titanium and aluminum, roughening of the surface electrode has been a problem.
Accordingly, a method of forming a p-type ohmic electrode using titanium and aluminum without an annealing process at high temperatures has been disclosed (for example, refer to Patent Document 1 or 2). More specifically, in Patent Document 1, a titanium-aluminum alloy (with no silicon (Si) included) characterized by exhibiting strong adhesion to a SiC single crystal that is serving as a base without undergoing a heat treatment has been described. Further, in Patent Document 2, a titanium-aluminum alloy (n-type SiC in Examples) interposed with a thin film composed of titanium carbide has been described. Both of these methods are capable of forming ohmic junctions at low temperatures without the requirement for a heat treatment at high temperatures.